1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus (hereinafter simply referred to as “image forming apparatus”) such as a copying machine, a facsimile, a printer, and a multi function peripheral which is of a multi-function copying machine and a scanning optical apparatus incorporated into the image forming apparatus.
2. Description of the Related Art
Generally, in the scanning optical apparatus incorporated into the copying machine, the laser beam printer and the like, a light beam emitted from a laser beam source is deflected with a deflector and collected with an imaging optical system such as an f-θ lens, a beam spot is formed on a photosensitive drum which is of an image bearing member, and the photosensitive drum surface is scanned by the beam spot. For example, in a tandem type color image forming apparatus in which four photosensitive drums corresponding to Y (yellow), M (magenta), C (cyan), and K (black) colors are arranged to form a multi-color image using Y, M, C, and K developers (hereinafter simply referred to as “toners”), laser beams emitted from laser beam sources are deflected, the photosensitive drums are scanned with the laser beams to form electrostatic latent images respectively. The electrostatic latent images are developed to four toner images with Y, M, C, and K development devices respectively, and the toner images are sequentially superposed and transferred onto an endless belt which is of an intermediate transfer member. The toner images transferred onto the intermediate transfer member belt are further transferred to a sheet. Then, the sheet is delivered to a fixing device, and the toner images are heated and pressurized to permanently fix the toner image onto the sheet.
For the tandem type image forming apparatus, there has been proposed an image forming apparatus in which one deflector are commonly used for the four photosensitive drums for the Y, M, C, and K colors.
In this case, the laser beam from the deflector toward the photosensitive drums is divided. In the laser beam separation method, there is a technique in which the laser beam is obliquely incident from each laser beam source to the deflector to provide an angle difference between the incident and reflected beams (obliquely incident optical system). In the laser beam separation method, there is also proposed a technique in which a height of the deflector is increased to provide plural deflection and reflection points in a vertical direction (for example, see Japanese Patent Application Laid-Open No. 2005-242131).
In the photosensitive drums in which the deflector is commonly used, because the plural laser beams pass through different heights in a sub-scanning direction on a single imaging lens, an amount of change in imaging position depends on the laser beam when an environmental temperature is changed, which results in color shift on an image. Because the deflector is located in the central portion of an optical box, it is necessary that plural reflecting mirrors be provided to increase an optical path length in the optical box. This causes a problem in that a structure is complicated as a whole.
On the other hand, in the tandem type image forming apparatus, there has been proposed a structure in which two deflectors and four photosensitive drums corresponding to the Y, M, C, and K colors is accommodated in a optical box. Each of the two deflectors is commonly used by the two photosensitive drums (for example, see Japanese Patent Application Laid-Open No. 2005-208176).
However, in this case, because each optical system is accommodated in a half of the total length of the optical box, when the optical path length is increased in the optical box, laser beams from the adjacent deflectors interfere with each other at the central position. Adoption of a structure in which the deflectors are forced away from each other leads to upsizing of the apparatus. When the number of reflecting times by the mirror is increased to increase the optical path length, the number of reflecting mirrors is increased to cause a problem of the complicated entire structure. The problem of the complicated entire structure is also generated in the case where the deflectors of the obliquely incident optical system are disposed while facing each other. This is attributed to the fact that, because a laser beam incident angle is not increased too much due to the color shift, the laser beams deflected by the deflectors interfere with each other or the laser beams are brought into close to each other.
Generally, a plane inclination correcting optical system is adopted when the deflector is used, there is a conjugate relationship between the deflector and the photosensitive drum surface in the sub-scanning direction, and an imaging optical system are commonly used in the Y, M, C, and K colors.
Therefore, it is necessary that the optical path lengths from the reflecting surfaces of the deflectors to the photosensitive drum surfaces of the Y, M, C, and K colors be matched with one another. Accordingly, in the scanning optical apparatus, each optical path is appropriately folded in the sub-scanning direction by the reflecting mirror and the like such that the optical paths from the deflection and reflection plane to the photosensitive drum surfaces are matched with another.
There is a limitation to a space which can be utilized in the image forming apparatus. In order to effectively use the limited space, desirably the optical path is shortened as much as possible outside the scanning optical apparatus.